Loud speaker improvement



Oct. 30, 1962 R MACDONALD 3,061,675

LOUD SPEAKER IMPROVEMENT Filed Sept. 9, 1958 2 Sheets-Sheet 1 INVENTOk Z4 5 Jameuiws/Vacdma/a MM MfM ATTORNEYS Oct. 30, 1962 Filed Sept 9, 1958J. R. MACDONALD LOUD SPEAKER IMPROVEMENT 2 Sheets-Sheet 2 ATTORNEYSUnited States Patent 01 3,661,675 LOUD SPEAKER IMPRQVEMENT James RossMacdonald, Dallas, Tern, assignor to Terras Instruments incorporated,Dallas, Tera, a corporation of Delaware Filed Sept. 9, 1958, Ser. No.766,036 SCl-aims. (Cl. 179--ll) The present invention relates to loudspeaker systems and more particulmly to a novel loud speakerconstruction and an associated circuit for developing negative feedbacksignals indicative of the distortion introduced into audio signals bythe speaker as a result of nonlinear movement of its diaphragm inresponse to audio signals applied thereto.

In recent years there has been an increasing emphasis on high qualityreproduction of sound by audio amplifier and speaker systems. Theconventional speaker employed in such systems comprises a voice coilwhich receives the audio signals to be reproduced and a permanentmagnetic construction for developing a magnetic held across the voicecoil so that when signals are applied thereto the coil is moved axiallyof itself by a magnitude and in a direction determined by the magnitudeand phase of the signal. The voice coil is secured to the central regionof a diaphragm of the speaker, which diaphragm has a generally hollowconical shape and which is rigidly secured at its outer peripheral edgesto the frame of the speaker. Movement of the voice coil in the fieldcreated by the permanent magnet of the speaker produces movement of thediaphragm and thereby produces movement of the air immediatelysurrounding the diaphragm so as to set up acoustic waves in this region.

Ideally, the diaphragm of a speaker should appear as a completely rigidbody so that the entire diaphragm moves as a single unit in response tosignals applied to the voice coil. As a practical matter, however, suchoperation is not achieved and at the lower frequencies considerableharmonic distortion is introduced into the audio waves generated by thespeaker. At the higher frequencies, standing Wave patterns are developedacross the surface of the diaphragm and further distortion is introducedinto the system. This latter phenomenon is commonly referred to as coneor diaphragm breakup. Additional difiiculties arise in the developmentof loud speakers that are intended to operate linearly over the entireaudio frequency band and to date efforts to produce such speakers havenot been fully successful.

It is an object of the present invention to provide a novel loud speakerconstruction and associated circuits tor producing signals which arerepresentative of the distortion introduced into an audio wave by theloud speaker producing the waves which can be fed as negative feedbackto the amplifier preceding the speaker.

It is another object of the present invention to provide a novel loudspeaker arrangement and associated circuit for producing a push-pullsignal indicative of the distortion introduced into an audio Wave by thespeaker producing the waves.

It is yet another object of the present invention to provide a novelloud speaker arrangement and associated circuit for producing twonegative feedback signals which may be applied to diiferent stages of anamplifier,

to a push-pull input stage of an amplifier or by slight ice modificationof the circuit to a single stage of an amplifier.

In accordance with the present invention, a diaphragm of a loud speakeris rendered conductive preferably by metallizing one of its twoextensive surfaces. The diaphragm is disposed between two metallicmembers which are substantially co-extensive with and in close proximityto the diaphragm. The diaphragm and metallic members form two capacitorswhose capacities are varied by movement of the diaphragm. An electriccircuit is employed to produce signals representative of the movement ofthe diaphragm and those signals are fed back to the signal source in anegative sense so as to minimize the distortion introduced into theacoustic waves by the speaker. Specifically, the diaphragm is connectedto ground and the two metallic members are connected in series throughdistinct very high impedance resistors to appropriate potentials.

The values of the two resistors are chosen such that the capacitors aresubstantially isolated from the source of potential at signalfrequencies and the only currents flowing in the circuit, once thecapacitors are initially charged, is the current required to compensatefor capacitor leakage.

One of the two capacitors formed by the loud speaker arrangementsubsists between the grounded, conductive, diaphragm and one of themetal members and the other capacitor subsists between the diaphragm andthe other of the metal members. The movement of the diaphragm increasesthe capacity of one of these capacitors and decreases the capacity ofthe other capacitor. Since the charges on the capacitors cannot vary atsignal frequencies due to the high impedances of the series resistors,an increase in capacity produces a reduction in voltage across thecapacitor in order to satisfy the equation while a decrease in capacityproduces an increase in voltage across the capacitor. By connecting thehigh impedance series resistors to equal positive potentials withrespect to ground the signals generated at the junction of thecapacitors and resistors are push-pull signals with respect to ground.If one of the resistors is connected to a positive source and one to anegative source both with respect to ground, the voltages at thejunction of the capacitors and resistors vary in the same sense withrespect to ground. In consequence the system is highly flexible and maybe employed to provide negative teedback signals for various stages ofan amplifier, a single stage or a push-pull stage thereof.

The signals generated in the circuits as a result of the rnovement ofthe diaphragm are indicative of the average movement of the incrementalelements of the entire surface of the diaphragm and therefore reflectany distortion introduced into the signal generated by the loud speakeras a result of improper movement of the diaphragm. The signals are fedback to the amplifier driving the speaker to modify the signal to thespeaker so that signal elements are produced by the speaker which helptocompensate for improper movement of the diaphragm.

It is, therefore, another object of the present invention to provide anovel loud speaker construction in which the diaphragm of the speaker isemployed as a movable condenser plate and in which an electrical circuitis employed to generate signals indicative of the movement of thediaphragm.

It is yet another object of the present invention to provide a loudspeaker having a metallized diaphragm and two rigid plates which conformin shape to the diaphragm and which are disposed on opposite sidesthereof and in close proximity thereto, all to said conductive elementsbeing insulated from one another so that they may serve as distinctcapacitor elements in an associated electrical circuit.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a vertical cross-sectional view of a speaker constructionsuitable as a dual capacitive element;

FIGURE 2 is a schematic circuit diagram of an electrical circuitincorporating the speaker of FIGURE 1 and employed to produce push-pullnegative feedback signals; and

FIGURE 3 is another schematic circuit diagram of an electrical circuitincorporating the speaker of FIGURE 1 and employed to produce push-pullnegative feedback signals.

Referring specifically to FIGURE 1 of the accompanying drawings, thereis illustrated a loud speaker arrangement which may be employed as oneelement of the apparatus of the present invention. The loud speaker isprovided with a conductive diaphragm 1 disposed between two aperturedand conductive plates 2 and 3, which conform, in general, to the shapeof the diaphragm 1 and are disposed in close proximity thereto butspaced therefrom. The diaphragm 1 and metallic elements 2 and 3 aregenerally conical in shape and the diaphragm may be fabricated from theconventional materials normally employed for such purposes. However, inaccordance with the present invention, this material is metallized oneither of its front or rear surface so that the diaphragm may beconsidered to be an electrically conductive member and is illustrated assuch in FIGURE 1. The members 2 and 3 are provided with a plurality ofapertures 4 and 5, respectively, so as to reduce'the air resistance tomovement of the diaphragm 1. The member 2 may constitute a wire screenhaving a one-quarter to one-half inch mesh. The apertures 5 in themember 3 should also be quite large and closely spaced and if the member2 does not constitute a part of the frame, it being such in theembodiment illustrated, it may also be a wire mesh.

The outer peripheral edges of the diaphragm 1 and the members 2 and 3are supported and maintained in fixed spaced relationship with respectto one another by means of an annular metallic support member 6 to whichthe diaphragm 1 and elements 2 and 3 are securely bolted. Specifically,the annular ring 6 is disposed against the right-hand peripheral surfaceof the apertured member 3 as viewed in FIGURE 1 while a non-conductiveannulus 7 is disposed in contact with its left-hand face, as viewed inFIGURE 1, so as to space the conductive member 3 from the diaphragm 1.The diaphragm 1 is disposed between the insulating disc 7 and a furtherannular insulating disc 8 which contacts its left hand outer peripheraledge. Disposed to the left of the insulating disc 8 and in contacttherewith is the outer peripheral edge of the apertured metallic member2. The diaphragm 1, metallic members 2 and 3, the annular frame member 6and the annular insulating washers 7 and 8 are provided with a pluralityof aligned apertures, only one being illustrated in FIGURE 1, which areadapted to receive the body portion of a bolt 10. The aperture in theannular frame 6 is threaded so that the screw may be threaded thereinto.A fiber or other insulating washer 11 is disposed between the head ofthe bolt and the apertured conductive member 2 so as to insulate themember 2 from the bolt 10 and therefore from the frame member 6. Theapertures in the diaphragm and the conductive member 2 are of sufficientdiameter that they do not contact the shank or body of the bolt 10 andif desired a fiber insulating cylinder may be disposed about the body ofthe bolt It) in the region of the diaphragm 1 and the apertured member2.

The apertured conductive members 2 and 3 are of relatively rigidconstruction whereas the diaphragm 1 is fabricated from conventionalloud speaker diaphragm materials and therefore, although being stifi,may be caused to move as a body between the members 2 and 3, in responseto the electric signals applied thereto as will become apparentsubsequently. The members 2 and 3 are apertured so that the diaphragmmay expel the body of air between it and one of the members 2 and 3rather than compress it. If the diaphragm were required to compress thisbody of air, it would become unduly loaded.

The diaphragm I is provided with a centrally disposed aperture 12, andterminates adjacent this opening in a rearwardly extending cylindricalsleeve 13. The cylindrical sleeve 13 is secured as by cementing to theouter peripheral of a hollow cylindrical collar 14. The inner peripheryof the cylindrical collar 14 is secured to a nonconductive, cylindrical,voice coil support .15 about which is wound a voice coil 16. The coil 16comprises many turns of line wire adapted to receive signal voltagesfrom a suitable amplifier. The left end, as viewed in FIGURE 1, of thehollow cylindrical sleeve 15 is provided with a conductive end wall 17which conforms to the shape of the apertured member 2 in the centralregion thereof and which is electrically connected to the diaphragm 1through the sleeve 14.

The apertured member 3 is also provided with a large centrally disposedaperture 18 of a greater diameter than the outer diameter of the sleeve13 which extends rearwardly from the diaphragm 1. The metallic aperturedmember 3 also has a rearwardly directed cylindrical sleeve 19 whichterminates in an annular shoulder 20 extending outwardly from the sleeve19 and perpendicular thereto. The sleeve 19 and shoulder 20 may be aseparate member from the plate 3 and may be secured thereto, as by spotwelding, etc. The cylinder 14 is provided at its rear edge with anoutwardly extending annular shoulder 21 which is disposed rearwardly ofthe shoulder 20 of the sleeve 19, but is aligned therewith. The annularshoulder 21 serves as a resilient spider 24 for the diaphragm of thespeaker and maintains the diaphragm in a predetermined position in theabsence of electric signals being applied to the voice coil 16. Thepredetermined position for the diaphragm is chosen to be preciselyhalf-way between the apertured metallic members 2 and 3. The conductiveflanges 20 and 21 are spaced from one another by means of a fiber orother insulating washer 22 and the members 2t), 21 and 22 are maintainedin contact with one another by means of bolts 31 which extend throughthese members and into a threaded aperture in an inwardly directedmetallic annulus 23 of the magnetic structure 24 of the speaker. Thebolt 31 is insulated from shoulder 20 so that the shoulders 20 and 21are not electrically connected.

The magnetic structure 24 further comprises a permanent magnet 25 havinga forwardly directed portion 26 of reduced diameter disposed within thehollow, cylindrical, voice coil support 15. The magnet 25 terminates atits forward edge in alignment with the forward edge of the inwardlydirected annulus 23 of the magnetic structure 24. The permanent magnet25 is bolted to a base plate 27 by means of a plurality of bolts, suchas the bolts 28, and the base member 2'7 is secured, as by a pluralityof bolts 2?, to a cylindrical member 30 which is secured to the annulus23 and extends rearwardly therefrom and perpendicular thereto. Theannulus 23, cylindrical member 30 and base plate 27 are all fabricatedfrom magnetic material so that the magnetic flux emanating from magnet25 is concentrated across the voice coil 16. The spider 24, which iselectrically conductive serves as an electric connection between thediaphragm of the magnetic structure 24 in addition to its function as adiaphragm positioning device.

It is apparent from the above description of the loud speaker apparatusof FIGURE 1 of the accompanying drawings that the conductive diaphragm 1and the apertured metallic members 2 and 3 are electrically insulatedfrom one another, and that these various members may be separatelyconnected in an appropriate electrical circuit. It is also apparent thatthe members 2 and 3 are rigidly supported with respect to one anotherwhereas the diaphragm 1 is readily movable with respect to the members 2and 3. in consequence the diaphragm 1 and the members 2 and 3,respectively, may serve as separate electric capacitors, the capacity ofwhich is varied in accordance with movement of the diaphragm 1 withrespect to the members 2 and 3. More particularly, the diaphragm 1 andmembers 2 and 3 serve as a physical push-pull arrangement since uponmovement of the diaphragm 1 toward one of the members the capacitybetween this member and the diaphragm is increased whereas the samemovement causes the diaphragm to move away from the other member anddecrease the capacity between the diaphragm and this latter member. Theloud speaker apparatus of FIGURE 1 is intended primarily to produceelectrical signals which are indicative of the acoustic wavesproduced bythe speaker and therefore these signals reflect the distortionintroduced into an audio signal in consequence of non-linearity ofmotion of the diaphragm 1 in response to signals applied to the Voicecoil 16.

A circuit which may take advantage of the novel construction of thespeaker of FIGURE 1 in order to produce push-pull negative feedbacksignals to reduce distortion in the speaker, is illustrated in FIGURE 2,of the accompanying drawing. Referring now specifically to FIGURE 2 ofthe accompanying drawings, those elements of FiGURES 1 and 2 which arecommon to the two figures bear the same reference numeral. The voicecoil 16 of the speaker is adapted to receive signals from an audioamplifier 32 in order to produce motion of the diaphragm 1 between theapertured plates 2 and 3. The diaphragm 1 is connected to a source ofreference potential, which for the purposes of illustration ishereinafter referred to as ground, and as a practical matter, theconnection would be made to some member of the magnetic structure 24- asillustrated in FIGURE 1. The apertured member 2 is connected to one endof a resistor 33, the other end of which is connected through a sourceof potential 34 to ground. The apertured member 3 is connected through aresistor 35 and a source of potential 36 to ground. The negativeterminals of both batteries are grounded but the circuit would operatein the same manner if the positive terminals of the batteries 34 and 36were grounded. The sources of potential 34 and 36 are illustrated asbatteries in the accompanying drawings, although it is not intended tolimit the source of potential to a specific unit since in a practicalembodiment of the invention, the sources of voltage will more thanlikely be derived from rectifier power supplies. The apertured plate 2is further connected via a lead 37 to a terminal 38 of the amplifier 32which constitutes one input terminal to a negative feedback circuit ofthe amplifier. The apertured conductive member 3 is connected via a lead39 to a second terminal 40 of the negative feedback circuit of theamplifier 32. A third terminal 41 of the negative feedback circuit ofthe amplifier 32 is connected to ground.

Under steady state conditions; that is, when no input signals areapplied to the amplifier 32 and therefore audio signals are not appliedto the voice coil 16 of the speaker, the diaphragm 1 is disposedcentrally between the apertured conductive members 2 and 3 and voltagesare developed between the diaphragm 1 and the members 2 and 3 equal tothe voltages of the sources 34 and 36, respectively. More particularly,the diaphragm 1 and the aperture conductive member 2. constitute a firstcapacitor which is connected in series with the resistor 33 and thesource 34. Similarly, the diaphragm 1 and apertured member 3 constitutea capacitor connected in series with the resistor 35 and the battery 36.So long as the diaphragm is not moving, steady state conditions prevailin the circuit and the voltages across the two capacitors are exactlyequal to the voltages of the sources 34 and 36, respectively. Theresistors 33 and 35 are of such a large value that they prevent theirrespective circuits from responding to variations in capacity of thespeaker arrangement but permit initial charging of the capacitors andsubsequent compensation for leakage of charge from the capacitors.

Upon movement of the diaphragm 1 toward the member 2 for instance, thecapacity, of which these elements form the plates, is increased andsince the charge stays essentially constant, the voltage across thecapacity decreases. At the same time that the diaphragm 1 moved towardthe member 2 it moved away from the member 3 anddecreased the capacitybetween the latter member and the diaphragm. Since the charge on thecapacity remains essentially constant and the capacity is decreased, thevoltage across the capacitor is increased.

As a result of the above cited operation, the voltage on lead 3'7decreases with respect to ground and the voltage on lead 39 increaseswith respect to ground so that the resulting feedback signal to theamplifier 32 is a push-pull signal with respect to ground. The signalappearing on leads 37 and 39 is applied across the terminals 38 and 4.6of the negative feedback circuit of the amplifier 32, and constitutes apush-pull, feedback signal which may be applied directly to a push-pullamplifier stage or to two distinct stages of the amplifier if sodesired.

The operation of the circuit when the diaphragm moves away from theapertured member 2 and toward the apertured member 3 produces the sameeffect as above except that the voltage on the lead 39 falls while thevoltage on the lead 37 rises and therefore the two possible conditionsof movement of the diaphragm I produce oppositely phased signals, thephase of which is dictated by the direction of movement of the diaphragm1 which in turn is dictated by the phase of the signal applied to thevoice coil 16. Therefore, the signal on the leads 37 and 39 is always ofsuch a phase as to constitute a negative feedback signal regardless ofthe phase of the output signal generated by the amplifier 32. Thefeedback signal generated by the circuit of FIGURE 2 is a directfunction of the movement of the diaphragm 1 regardless of whether thediaphragm moves as a unitary stiif body as at low frequencies or movesas a discontinuous body as when high frequencies are applied to thediaphragm which produce high frequency break-up. Regardless of themanner in which the diaphragm 1 moves, the total change in capacitybetween the diaphragm and one of the elements 2 and 3 is a function ofthe movements of each increment of the diaphragm 1 and therefore thesignal generated represents the average movement of the diaphragm andincludes signal elements indicative of distortion thus introduced.

In the arrangement illustrated in FIGURE 2 of the accompanying drawings,the signals produced are push-pull signals which, as indicated above,may be applied directly to a push-pull stage of the amplifier or may beapplied to two distinct stages of the amplifier which have signals thatare out of phase applied to the control element of the respectivestages. If it is desired to provide feedback signals to two stages of anamplifier having input signals of the same phase applied to theirrespective input grids, then a provision must be made to produce signalson the leads 37 and 39 having the same phase with respect to ground.

Referring specifically to FIGURE 3 of the accompanying drawings, thereis illustrated an embodiment of the invention in which signals areproduced on leads 37 and 39 that are of the same phase with respect toone another. Those elements of FIGURE 3 which are common to both figuresbear the same reference numerals as FIG- URE 2. In this embodiment ofthe invention a resistor 42 is connected between a terminal 43 which ispositive with respect to ground and a terminal 44 which is negative withrespect to ground. The end of the resistor 33 re mote from the member 2is connected to a point on the resistor 42 which is positive withrespect to ground whereas the end of the resistor 35 remote from themember 3 is connected to a point on the resistor 42 which is negativewith respect to ground to the same extent that the point to which theresistor 33 is connected is positive with respect to ground.

In operation of this embodiment of the invention, and it is assumed thatthe diagram 1 moves toward the member 2, upon movement of the diaphragmthe voltage be tween members 1 and 2 is decreased and therefore thevoltage on the lead 37 decreases with respect to ground. The voltageinitially on the lead 39 appearing at lead 3% is negative with respectto ground and upon movement of the diaphragm ll away from the member 3the voltage across this capacitor increases and therefore the voltage onthe lead 39 decreases further with respect to ground. As a result, thevoltages on the leads 37 -and 39 both exhibit a decreasing magnitudewith respect to ground and therefore the voltage on these leads are thesame phase, and the operation of this circuit is the opposite to that ofthis circuit of FIGURE 2.

The circuit of FIGURE 3 is essentially the same as that of FIGURE 2except for the use of the voltage divider instead of the batteries 34and 36. The only modification of the circuit of FIGURE 2 that it isrequired to produce the circuit of FIGURE 3 is to reverse the battery 36so that its positive terminal is grounded and the negative terminal isconnected to one end of the resistor 35. The voltage dividing resistor42 of FIGURE 3 is employed merely to illustrate an alternativearrangement to the one utilizing the batteries illustrated in FIGURE 2.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

What I claim is:

1. In combination, a loud speaker having a conductive diaphragm, afirst, apertured conductive member disposed in a plane parallel to theplane of said diaphragm; said first apertured member being arranged inclose proximity to and spaced from one side of said diaphragm; a second,apertured, conductive member disposed in a plane parallel to the planeof said diaphragm; said second, apertured member being arranged in closeproximity to and spaced from the other side of said diaphragm; saiddiaphragm and said apertured members being electrically insulated fromone another, means for electrically connecting said first aperturedmember in series with said diaphragm and a source of voltage; means forelectrically connecting said second apertured member in series with saiddiaphragm and a source of voltage; and means for producing two electricsignals each being indicative of the variation in voltage across saiddiaphragm and said first and second apertured members, respectively.

2. In combination, a loudspeaker including a frame; a conductivediaphragm having its outer peripheral edge secured to said frame; firstand second, apertured, con ductive members having approximately the sameshape as said conductive diaphragm; said first and second memberssubsisting in planes approximately parallel to the plane of saiddiaphragm and being disposed on opposite sides of said diaphragm inclose proximity thereto and spaced therefrom; said diaphragm and saidapertured members being electrically insulated fromone another; meansfor electrically connecting said first apertured member in series withsaid diaphragm and a source of voltage; means for electricallyconnecting said second apertured member in series with said diaphragmand a source of voltage; and means for producing two electric signals,each being indicative of the variation in voltage across said diaphragmand said first and second apertured members, respectively.

3. In combination, a loudspeaker having a conductive diaphragm; a first,apertured, conductive member disposed in a plane parallel to the planeof said diaphragm; said first apertured member being arranged in closeproximity to and spaced from one side of said diaphragm; a second,apertured, conductive member disposed in a plane parallel to the planeof said diaphragm; said second apertured member being arranged in closeproximity to and spaced from the other side of said diaphragm; saiddiaphragm and said apertured members being electrically insulated fromone another; a pair of impedances, means for connectin said firstconductive diaphragm in a first series circuit including said diaphragm,said first apertured member, one of said impedances and a source ofvoltage, means for connecting said diaphragm in a second series circuitincluding said diaphragm, said second apertured member, the other ofsaid impedances, and a source of voltage and means responsive to thealternating voltages across each of said members and said diaphragm,said impedances being of such a high value as to substantially preventcurrents from flowing at audio frequencies.

4. In combination; an amplifier having an input circuit, an outputcircuit, and a push-pull, negative feedback circuit having two inputterminals; a loudspeaker having a conductive diaphragm; a first,apertured, conductive member disposed in a plane parallel to the planeof said diaphragm, said first apertured member being arranged in closeproximity to and spaced from one side of said diaphragm, a second,apertured, conductive member disposed in a plane parallel to the planeof said diaphragm, said second apertured member being arranged in closeproximity to and spaced from the other side of said diaphragm, saiddiaphragm and said apertured members being electrically insulated fromone another; a pair of impedances, means for connecting said conductivediaphragm in a first series circuit including said diaphragm, said firstapertured member, one of said impedances and a source of voltage, meansfor connecting said diaphragm in a second series circuit including saiddiaphragm, said second apertured member, the other of said impedances,and a source of voltage and means for connecting said first and secondapertured members to different ones of said input terminals of saidnegative feedback circuit, said impedances being of such high valuesthat said circuits are substantially nonresponsive to signals at audiofrequencies.

5. In combination; a loudspeaker including a frame; a conductivediaphragm having its outer peripheral edge secured to said frame; firstand second, apertured, conductive members having approximately the sameshape as said conductive diaphragm; said first and second memberssubsisting in planes approximately parallel to the plane of saiddiaphragm and being disposed on opposite sides of said diaphragm inclose proximity thereto and spaced therefrom; said diaphragm beingconnected to a point of reference potential; said diaphragm and saidapertured members being electrically insulated from one another; a pairof impedances, means for connecting said conductive diaphragm in a firstseries circuit including said diaphragm, said first apertured member,one of said impedances and a source of voltage, means for connectingsaid diaphragm in a second series circuit including said diaphragm, saidsecond apertured member, the other of said impedances, and first sourceof voltage, the end of said source of voltage remote from said one ofsaid impedances being connected to said point of reference potential,means for 9 10 connecting said diaphragm in a second series circuit inReferences Cited in the file of this patent eluding said diaphragm, saidsecond apertured member, UNITED STATES PATENTS the other of saidlmpedances and a second source of voltage having its end remote fromsaid other of said im- 3? Harry 30, 1945 pedances connected to saidpoint of reference potential; 5 2,400,993 PS? y 1946 and meansresponsive to the variation in voltage between 2,683,861 Vlerhng et a1 13 1954 said point of reference potential and said first and second2,755,343 Levy y 17, 1956 members, respectively, said impedances beingof such high 2,857,461 Brodie t. 21, 1958 values that said circuits aresubstantially nonresponsive to 2,864,899 Parker Dec. 16, 1958 signals ataudio frequencies. 10 2,896,025 Janszen July 21, 195.

